1. Field of the Invention
The present invention relates to a display control system and, more particularly, to a display control system used for a computer such as a personal computer or a workstation.
2. Description of the Related Art
As a conventional display control system used for a computer such as a personal computer or a workstation, a system having an intermediate resolution of about 640.times.480 dots has been widely used. In recent years, however, with an increase in necessity to use an advanced graphical user interface, demands have arisen for a high-resolution, high-performance display control system.
In order to perform a high-resolution graphic display operation, a large-capacity image memory must be used as a frame buffer for holding image data such as character and graphic pattern data, and the contents of the image memory must be rewritten at high speed.
For this reason, in general, the memory bus width of a bus connected to the data input/output port of an image memory is set to be as large as 32 bits or 64 bits. By using such a technique, a large number of pixels are updated with one memory access.
The access speed of a host CPU or a drawing coprocessor with respect to an image memory varies depending on an instruction executed by the processor. More specifically, memory access instructions issued from the processor include a normal memory access instruction and a string move instruction (continuous data transfer instruction). The string move instruction (continuous data transfer instruction) allows a data string having continuous addresses to be read/written at high speed with one access. The access speed based on this instruction is very high.
As described above, the access speed of the processor is not constant. Therefore, when the processor is to access the image memory, synchronization must be established with respect to an access operation of image memory for every access, and the processor is kept in a wait state until the synchronization is completed. Especially in a data read operation, since read access to the image memory is started after a read request is issued from the processor, the processor is kept in a wait state without being released from access processing until the corresponding read data is confirmed. This is a main factor which causes a deterioration in the performance of the processor.
In a conventional system, when a processor is to access an image memory, synchronization must be established with respect to an access operation of the image memory for every access, and the processor is kept in a wait state until the synchronization is completed.